External SDD Forming issues

What year model is the iMac?

Identify your iMac model - Apple Support

What 1T SSD are you trying to use?

(brand and model # are very important)

How is 1T SSD connect to the old iMac?

(what USB cable and adapter if one was required)

________

1) You should never use the same external drive for Time Machine backups and random file storage.

As Time Machine consumes drive space, it can corrupt the random storage.

2) You should choose an external drive that is best suited for the task.

A slower external HDD drive is fine for Time Machine.

A faster external SSD drive is better for file storage.

3) You should also choose an external drive that is best suited to the iMac.

A fast USB-C external SSD is not going to run fast plugged into the slow USB 2.0 port of an old iMac.

iMac Model - iMac (21.5 inch, Mid 2011 Version 10.10.5 OS X Yosemite Serial No C02GFACXDHJF.

The SSD Brand and Model is not identified on the Case, Was purchased On Line.

It is connected to my iMac using a USB cable to one of the 4 USB ports in the back of my iMac.

1) Are you WGT52?

If so, then you should log back into the community using that ID.

2) The make and model of the SSD is important here.

Look on the package or post a link to the external SSD that you purchased.

3) The USB ports on that old 2011 iMac are only USB 2.0.

The speed of the SSD will be severely cripple by the lower USB 2.0 bus speed of that iMac.