before buying an imac mid 2011 used what shoud i check

I would not buy one that old. I would recommend buying an Apple Refurbished iMac directly from Apple. These are identical to a new machine, can save hundreds of $$, come with a 1 year warranty that can be extended to 3 years with AppleCare, have no downside.

You can find them at: https://www.apple.com/shop/browse/home/specialdeals/mac

If you don't see what you want, check back often as inventory can change by the hour.

If you buy a used computer, yes even an iMac because you NEVER know for sure what you are buying.

Fact of life, hard drives will fail. At being 6 years old, that

HDD is beginning to live on borrowed time. Replacing a

hard drive in a 2011 iMac is very difficult (read expensive)

and requires a special drive with a special temperature

sensor or a special adapter cable that has such a sensor.

However, 2011 iMacs can be booted from an external drive.

Unfortunately, the only reasonably fast interface in that model

would be the Thunderbolt interface, which being quite fast,

will have the pricier of external drive interface options. Firewire 800

can be used but will not be all that fast. And since the 2011 only

has USB2, that would be excruciatingly slow for a boot drive.

With that said, I would only purchase if I can view it in person

to make sure there are no screen issues and be able to run Apple's

utility for self checking hardware and see the results in person.

There are reputable vendors out there that sell fully tested and

warranted used iMacs. One that comes to mind is OWC. They

not only thoroughly test their used Macs, they have a 14 day

no questions asked return policy and a limited 90 day warranty

(parts and services for defect repairs).

https://eshop.macsales.com/shop/Apple_Systems/Used/Macs_and_Tablets

I am running my 2009 iMac from an external FW800 enclosed SSD.

Until you have tested or seen this working on an older Mac, you have NO idea how much faster a older IMac can run on on an external SSD over FW800!

You'll ger an extra 10-15 Mbps throughput through the FW800 connection.

I am averaging about 94 Mbps sec data throughput versus my normal external spinning hard drives which peak at about 75-78 Mbps data throughput.

But the responsiveness of ny iMac on this external SSD is pretty substantial and a pretty noticeble uptick when compared to my internal HD and even booted from a normal external FW800 mechanical hard drive!

Admittedly, my internal mechanical hard drive is on its way out after 8 years, but is still working enough where I'll keep using for as long as I can for just storage.

I have 3 other larger external FW800 connected mechanical drive that I store my data to and bot from, also, and they are NO where near as fast as the externally connected SSD.

I think a lot of delay and data throughput issues of mechnical hard drives are the general speeds of the mechanics.

The speed of the platters, the seek times of the heads and read/write speeds from the disc and through the FW800 connection must cause the data throughput to be slower.

Something definitely to be said for storage and boot drives with NO moving mechanical parts!

I am. very happy camper!

Been using this SSD drive as my iMac's main boot drive now for a little over 3 months!

It just works and feels like a brand new Mac running from this SSD.

I am just not sure about the longevity of the current crop of SSDs VS the "old school' conventional mechanical spinning hard drives, though.

I guess over time, SSDs will get cheaper, better and more reliable just like with standard hard drives.

This SSD is a 480 GBs drive that I could finally afford to purchase. The 1 and 2 TB SSDs are still too expensive, right now.

I have approx. 120 GBs of free data space left on that SSD after cloning my OS X system off of my iMac's internal drive!

Anyhoo..

Someone said that this year and model iMac is too old?

I don't have any issues purchasing an older 2011 27 inch screen iMac.

My next used iMac "upgrade" (maybe soon) will definitely be a fully loaded up/optioned out 2011, 27 inch screen iMac.

I need an iMac still capable of running OS X 10.8 and 10.9.

It is going to have the i7 CPU, 2 GBs VRAM GPU chip, either the standard 2 or 3 TB hard drive OR, if I can find one, the 256 GBs SSD installed, at least 16 GBs of RAM, looking for one with 20 or 24 GBs of RAM.

This time out I am going to get a used 2011, 27 inch iMac that is pretty much maxed out!

I didn't do enough research last time out and purchased an underpowered iMac for my needs.

NOT happnin' this time.