Harddrive from old computer as external drive for new computer

An a example of a 'bare wire kit' (avoids, or to use instead of enclosure)

for an external drive to connect and have independent power, & data;

~ an idea presented as "universal drive adapter USB bare wire adapter."

(Includes power adapter, wires, and various connectors to be flexible..

This one goes by name NewerTech universal drive adapter from OWC.

..Online manual shows product & parts, with instructions in PDF.)

Can be also useful to run full-system clone; to power up external OS.

Also works to consider, while making due with other workarounds.

Good luck & happy trails!

🌞🌿⛵️🎣🌻🐢⛄️🌤🌎

If you're planning on using this as a true external drive then you probably want more than the "bare wire" connector discussed by K Shaffer. I myself prefer the NewerTech Voyager docking station which lets me easily swap bare drives. I have one used 95% of the time by a backup drive and then 5% of the time when I put an archive drive on it for storing little used data. Alternatively you can buy a proper enclosure.

I cannot think of any offhand, though I would be careful about doing something such as using any of Disk Utility's repair features from an older OS on a newer OS drive. You may also have to be careful about which versions of apps open which files. You don't want to open something with Pages in Catalina and then try to open it with Pages in High Sierra. The biggest culprit will be iTunes which was rebranded in Catalina. I have a dual boot system but I very much restrict the use of the second OS to very specialized and restricted tasks.

I am a bit surprised at what you say about the SSD not working. From what I have read it should just take the standard SATA connector that works with any SATA connection. I have a brand new SSD that fits on my 5 year old Voyager docks that also accept my 8 year old HDDs.

The operating system doesn't matter, it's the format that does. Now if you were using El Capitan and Catalina things would be different since with High Sierra Apple started using APFS on its drives. A computer booted to ElCapitan can't see a Catalina drive.

The place where you may have problems is trying to access files in directories with permissions. I get kind of lost with that myself and I suspect Apple's ever increasing security mindedness doen's make it easier as go up the OS ladder. My solution is I put any data files I want to use on a completely separate partition that has no ownership on it and can be read by any system or user (only me on this one).

1. Yes.
2. Yes, unless there are permissions problems. As I said before I avoid those altogether by having data files on drives with no permissions so I don't have firsthand experience apart from memories from the dim past of getting access to files in other user accounts on a drive.
3. Yes and yes.

High Sierra was the first to implement APFS drive formatting. I believe it was optional but on later OS versions it was required. APFS post-dates El Capitan which means if you attach an APFS drive to El Capitan it won't recognize it. So yes, this will be a problem unless when using El Capitan you do not care about using any files on the Catalina drive.

Okay, now it could get horribly complicated because there are all kinds of options depending upon what you want to do in terms or partitioning and using files on which drives etc. In my case I have dual boot. High Sierra is my main system and is on an external APFS SSD. My internal drive is now a data only drive on a HFS (old Macstyle) HDD. I also have a Mojave partition on the SSD but I only have that for using to backup my iPhone, nothing else. I also have a folder on my internal where I put applications to be used by any system to which I boot. Okay, actually I also have an El Capitan partition on the internal drive just in case something I have doesn't like High Sierra. I keep it on the internal because although the SSD is faster, my understanding is it works better with APFS but El Capitan would require HFS and that I use on my HDD because APFS can he hard on HDDs.

If I boot to El Capitan and want to go back to High Sierra I have to hold down the option key while booting because I cannot choose HS from the El Capitan startup preference pane because it does not see HS on APFS.

Okay, I'll let you ponder all that. :-)

MexRaider:

These exemplary accessories will work adequately w/

your various technologies; aside from citing OWC as

potential source. I've a cross-section of mid-modern to

antique Macs; to keep some older vintage nearby as a

good idea rather than trying make recent run old OS.

Pair technologies with best suited, & you've not gone wrong.

Just try to keep spare place to 'weather-in-place' older ones.

Out of dozens, I've still a few keepers; they ran last I'd checked.

I am not closely familiar with the 2011 MBP but you probably can, yes.