Why does font book keep disabling fonts

I wish there had been some advice given here regarding your question. I have the same issue, where Arial Regular and Arial Bold (but NOT Arial Italic or Arial Bold Italic) keeps getting turned off in Font Book. I turn them on, and it's fine. But the next time I shutdown and boot up, they are turned off again.

I don't think this is a Mavericks issue, because this has happened to me before a couple of times on different Macs. One time it was Georgia, another Verdana, and this time it's Arial. In my cases, it's always been a core system font which makes it particularly annoying. Font Book is a shoddy application by reputation, so I place the blame there. If you want to know how my previous font issues were resolved, I have no idea. Each time, my computer must have decided to finally stop turning the font off, but I have no idea why. So I'll just wait it out again unless someone has an idea.

jessie_vp, do you have another font management app installed? The only common factor for me in these cases is that I also have Suitcase Fusion installed. But the system fonts aren't even in Suitcase's library, so I don't see how it could conflict?

To start with, never have more than one font manager on any system at the same time. They will fight each other over control of which fonts are enabled/disabled. Pick one and completely remove any others. That includes removing Font Book if your preferred font manager is Suitcase, FontExplorer X Pro, or other app.

From there, you have to go down the line of possibilities. From easiest to test and fix, to the most time consuming:

1) Corrupt fonts will damage Font Book's database. And rather easily at that. You can clear it, have the fonts working for a day or two, and then the damaged fonts toast the database again and you're back to a non functioning Font Book. To completely reset Font Book:

Restart your Mac and immediately hold down the Shift key when you hear the startup chime to boot into Safe Mode. Keep holding the Shift key until you see a progress bar towards the bottom of the screen. You can let go of the Shift key at that point.

OS X asks you to log in (you will get this screen on a Safe Mode boot even if your Mac is set to automatically log in). Let the Mac finish booting to the desktop and then restart normally. This will clear Font Book's database and the cache files of the user account you logged into in Safe Mode.

When you launch Font Book after booting normally, a new database will be built. If the problems return, you likely have bad fonts on your system somewhere.

2) So the next step is to get the original fonts back on the system, and only the original OS X supplied fonts. Move all fonts out of your user account Fonts folder to a temporary folder on the desktop. Move all fonts out of the root /Library/Fonts/ folder to another temporary folder. Download Pacifist. Follow the instructions at the bottom of Font Management in OS X to reinstall just the fonts OS X supplies.

For Mavericks, that would require first downloading the full installer .dmg file through the App Store. When the installation procedure starts after the download is complete, stop it by pressing Command+Q. Note: the installer is supposed to remain on the drive in the Applications folder when you quit the installation, but it sometimes gets deleted anyway and you have to download it all over again. So before you quit the app, just leave the initial installation screen up, go back to the desktop and copy the installer somewhere else, then quit. If you end up with both, then just delete one or the other. Now use Pacifist to reinstall OS X's supplied fonts, it will overwrite all of the fonts on the drive will all new, fresh copies.

Repeat the steps above to clear Font Book's database and font cache files.

If, with only freshly installed OS X supplied font on the system you still have Font Book randomly turning fonts on or off, then the OS needs to be reinstalled.

I may receive some form of compensation, financial or otherwise, from my recommendation or link.

I hadn't considered disabling Font Book as an option because it felt like a core OS application and I didn't want to mess with it. This led me to find this support doc on Extensis' website: http://www.extensis.com/support/knowledge-base/suitcase-fusion-5-recommendations -os-x-10-9-mavericks/ So I will try disabling Font Book. Thanks! I hope this helps the OP as well.

I hadn't considered disabling Font Book as an option because it felt like a core OS application

It only seems that way because since Lion, any app OS X installs is treated as some sort of immutable object. Yes, some are important, but the OS will also tell you Chess is a critical application, and cannot be removed. Chess? I don't think so. Font Book is no different in that regard. It's just another font manager that has no special importance to the OS itself.

In order to disable Font Book, you must remove it. See the same article I linked to above. The last part of section 6 explains how to completely remove Font Book.

Let me make it abundantly clear, for the record: even Apple's preinstalled TTC (TrueType Collection) fonts are not exempt from this bug. I tried a clean reinstall of the OS, without adding third-party fonts, and still encounter this error for which I've yet to locate a solution.

I don't even have to clear caches and reboot for fonts I re-enable to become disabled: it happens every time I quit the application. Sometimes I dont even have to quit before they become disabled again. Half of Apple's own font library is greyed-out. And even the families that are not greyed-out only have the Bold weight enabled. Baskerville.ttc — the font used by Apple's Dictionary app — is one such example. It makes the entire application look horrible.

This is infuriating!

I am running Yosemite 10.10.2 (14C1514) on a MacBook Pro only three months old, with no other font managers installed.

Apple's supplied fonts (most of which don't actually come from Apple) aren't the problem. Other than a known bug in the supplied version of Palatino (superscript doesn't work right above 12 points), they all work as expected. I also won't use Font Book. It's one of the very first apps that gets deleted from any Macs we use.