Late 2013 iMac - issue using 2nd external SSD

[gramps]

I have a late 2013 iMac 21.5" i5 running Catalina.
Months ago I fitted a 480GB external USB SSD (Pioneer APS-XS03) and cloned the internal HD, making the external SSD the boot drive and everything worked brilliantly.
This week I got another identical USB SSD to use for photo storage - issues!
On bootup with the additional SSD fitted the iMac struggles to boot and walks through treacle from then on ... it's basically unusable!
Remove the 2nd SSD and it boots up as good as it has ever done.
I've checked and the original SSD is still showing as the selected bootup disk.
It's as though the iMac is confused at having two SSD's the same but I can't see how that could be the problem.
Any ideas welcomed.

 

[ancient_mariner]

It's likely to do with port sharing and use of bandwidth between the 2 drives. Are they daisy chained or plugged into separate ports?

 

[gramps]

It's likely to do with port sharing and use of bandwidth between the 2 drives. Are they daisy chained or plugged into separate ports?

Thanks Toni, they are plugged into seperate USB posts.

 

[ancient_mariner]

Thanks Toni, they are plugged into seperate USB posts.

Leave them both plugged in overnight with the machine running.

I just replaced a drive in this machine with a 'full' drive and it spent better part of an hour running at about 50% load & spinning up the fans. It was trying to index the new drive, and I wonder if OSX could be buggering about with the new drive in some way too, filling it with sidecar files or something.