Are External Drives Affected by Write Permissions?

Yes, particularly in Windows 7 external write permissions are often disabled. While you may or may not encounter the problem it isn't really a big deal if you do.

There are two basic ways to fix the issue:

Login as an administrator on the computer. Open regedit.exe by typing that in the start search box. Navigate to this key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\StorageDevicePolicies
You might need to create the key StorageDevicePolicies if it does not exist yet.
Select the StorageDevicePolicies key with a right-click into the right window.
Select New > Dword (32-bit) Value from the menu and name the new entry WriteProtect.
Double-click the new WriteProtect key and change the value to 0. If you want it to be read only you can change it back to a value of 1.
Save the key changes by clicking OK after each.
Restart your computer and then attach and test the USB device.

There is one other thing that I use if that fails in Windows 7 (older versions use a different syntax), and it has always worked. Plug the device in, perhaps into a different USB port, then use diskpart to change its attributes:

Open the command prompt box with right click and run as administrator

Type each of these commands one at a time in this order:
diskpart
list volume
select volume n (where n = the problem volume)
attributes disk clear readonly
attributes volume clear readonly

After each of the last two you should get a response that it was successful.