How to upgrade iMac 21” 2009 for High Sierra

Dannysb211 wrote:

Thanks ever so much every one, youve been amazing, I finaly mananged, I downloaded OS X 10.6.8 to Windows lapi, and installed it my imac 2009 remotely, and its all working fine now, am trying to upgrade to 10.13, any advise please?

I believe you're going to need to upgrade the OS in two (or more) steps.

To download High Sierra, you're going to need versions of Safari and the App Store that are new enough to work with the App Store link for High Sierra in the Support article. That probably means upgrading to El Capitan or to Sierra.

Apple makes the El Capitan and Sierra installers available in the form of .DMG (disk image) files. If need be, you could download those files using some computer other than that old iMac, put them into a USB drive (e.g., flash drive) or SDHC/SDXC memory card, and then copy them from there onto the old Mac. You'll need the Mac itself for opening the disk image and running the installer application within.

Caution: When I upgraded my Late 2009 iMac straight from Snow Leopard to El Capitan, then to Sierra, then to High Sierra, one of those upgrades (probably the first) took so long I thought the machine was dead. I had to let the thing run overnight or even longer for the update to finish. After one of the updates (same one? not sure …) the machine staggered very slowly to its feet and ran like molasses for a couple of days. I'm guessing Spotlight decided that its old index was out of date, and that it needed to build a new one from scratch.

If you have any bootable clone backups of your startup disk, now would be the time to use them.

--------------------

A 21.5" Late 2009 iMac would have come with Snow Leopard, and a set of discs for installing Snow Leopard. (This was in the days before Recovery partitions and Internet Recovery.). If you have those discs, you could install Snow Leopard and then try upgrading to High Sierra.

Once you have Snow Leopard installed, you can probably upgrade to High Sierra in two steps:

1. Download the .DMG file containing the Sierra installer (using another computer if necessary). Open (mount) the .DMG file on the iMac and run the Sierra installer.
2. Using the versions of Safari and the App Store in Sierra, upgrade to High Sierra using the App Store link.

How to download and install macOS - Apple Support

--------------------

If you're trying to get Snow Leopard back to the point where you can try to use the Mac App Store, that might be a little bit more involved. The Snow Leopard recovery discs would have contained Mac OS X 10.6.1 or 10.6.2, and the Mac App Store didn't make an appearance until 10.6.8.

There are a bunch of Snow Leopard patches at Apple - Support - Downloads. Two of the really important ones are

Mac OS X 10.6.8 Update Combo v1.1

Mac App Store Update for OS X Snow Leopard

Apply them in that order. The first brings Snow Leopard up to 10.6.8 from any previous 10.6.* version. The second patches the App Store support in 10.6.8 to "ensure future compatibility of the Mac App Store". Once again, these may not be necessary if you can get any version of Snow Leopard, and the .DMG with the Sierra installer, onto the machine.

If you are using a wireless keyboard, plug in a wired USB keyboard.

If you have the original Install Disc set that came with that iMac, then try using Disc 1.

If the USB stick is good, plug it in and startup holding the Option key for the Recovery options.

If it was running a later macOS version like El Capitan or High Sierra, try Option-Command-R for Internet Recovery.

see > Use macOS Recovery on an Intel-based Mac - Apple Support

I didn't try upgrading to any of the intermediate versions between Snow Leopard and El Capitan, so I don't know if adding extra intermediate upgrades would help.

I would hope that one at a time would not be necessary. (You'll have to skip 10.9 (Mavericks) in any event, since, for some unexplained reason, Apple does not offer a download of its installer.)

Once you have a recent enough version of macOS installed to be able to use the High Sierra link, be sure to have Safari selected as your default browser, and to use it to open the Support article. (The link might not work if you use another browser – and the error message you get could be very misleading, leading you to think that Apple had stopped offering the High Safari installer.)