eSATA External HDD only recognised once per port per restart

I've installed MaxPower eSata 6G PCIe2.0 card into my Xserve2,1

I have a VANTEC NexStar Hard Drive Dock connected to this.

I have 3 x 3TB HDD's. I copy files to this one HDD at a time and rotate the drives every week for Offsite backups.

Issue is: HDD works fine - recognised in Disk Utility, mounts automatically.

When swapping drives, I unmount current drive and insert the next drive. This is NOT recognised - not mounted in Finder, doesn't show up in Disk Utility, can't find any reference to it in System Profiler.

I then unplug the eSATA cable from the first eSATA port into the second eSATA port - this works fine and HDD mounts automatically. Next week, try the same process, but now neither eSATA port works.

Restart and both work fine again, so it appears that a HDD can only be used on one eSATA port per restart. I don't want to restart my server every second time I swap a drive.

I have asked the company where I've bought the eSATA card from and they suggested a PRAM reset which I've done and which hasn't solved anything. Issue persists.

I have also disconnected eSATA card and reseated. Issue persists, plus it's probably a software issue rather than a hardware issue.

Tried with different hard drives - issue persists.

Tried with different HDD docks (although same model) - issue persists.

Had the issue for months and keen to resolve.

Any ideas? Tips on how to troubleshoot? Which CLI commands may give me relevant troubleshooting information?