Don't see how that be be related, sometimes users pass-trough a controller by mistake to a VM, so when the VM starts all disks connected there are lost to Unraid, but that's not the case here.
OK, now you can move the data to another disk in the array (or UD), like mentioned that disk won't mount in the array because Unraid would complain of an invalid partition.
Disk dropped offline before, this is usually a power/connection problem, check/replace cables and if the emulated disk is mounting and contents look correct you can rebuild on top.
Unraid can't do anything about that, it's limited by the disks speed, compare for example 8 and 16TB disks, capacity doubled but max speed went from around 225MB/s to 275MB/s for the fastest models.