Can I remove the drive letter on the SD card I'm using for Readyboost?

"Handy little tool called HideDrive does the trick. Tested and working on my own computer with Readyboost on a USB stick. You can still access the drive, even though it is hidden from Explorer which is exactly what I wanted to do too, just never got around to it. Word of warning, the tool restarts your computer when you hit apply.

If you are feeling adventurous, you could also edit the registry directly. Basically, navigate to Computer\HKEY__CURRENT__USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
and edit NoDrives to a decimal value using the chart below. Simply add up the numbers for the drive letters you want to hide. So to hide drive I:, set it to 256.

Drive Letter DECIMAL Value
A 1
B 2
C 4
D 8
E 16
F 32
G 64
H 128
I 256
J 512
K 1024
L 2048
M 4096
N 8192
O 16384
P 32768
Q 65536
R 131072
S 262144
T 524288
U 1048576
V 2097152
W 4194304
X 8388608
Y 16777216
Z 33554432"

Honestly, and I know this isn't an answer, but why not just invest in more RAM? SD is traditionally slow with writes, and readyboost writes to it a lot, so you may be going even slower than if you didn't have it at all and the OS paged normally.