No really permanent, but late alphabet best
All versions of Windows feel that they can rearrange things to suit themselves. On the other hand, Windows seems to leave things stable unless it "needs" to use them differently. Assigning a letter in the P, Q, R, S, T range greatly reduces the chances that Windows will "need" the letter for something else.
Details you may not need to know:
Windows leaves the A and B letters for floppy drives, regardless of whether there are any present.
Windows assigns the "first" internal hard drive letter "C". It determines "first" by iterating through IDE, SATA (and older) drives starting with first interface Master, then second interface master, then first interface Slave, then second interface Slaves. Then it goes through SCSI drives in SCSI number order (and yes, it uses secondary SCSI numbers as well if present). Some system BIOS software allows SCSI to be set first. Only then does it iterate through USB or firewire drives (I'm really not sure which goes first), choosing in order by USB tree (128 possible per controller, but controllers in order as set up by the system).
CD or DVD drive letters are set up AFTER hard drives, also in interface order.
As you can see, adding new hard drives tends to displace older ones.
A complicated example:
B - reserved for missing floppy
C - IDE hard drive first master
D - IDE hard drive second master
E - IDE hard drive first slave
F - CD drive second slave
G - SCSI hard drive ID 2
H - SCSI hard drive ID 3
I - SCSI CD drive ID 5
J - USB hard drive on port 1
H - USB hard drive on port 4
I - USB CD drive on port 3
J - USB thumb drive on port 2 (note has lower priority than hard drives or CDs despite lower port number)
N - networked hard drive (if it has a letter, you set it up, or the network admin did) note out of sequence lettering
P - personal network share that you set up
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