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Q: When we assign drive letter to one partition by using EaseUS Partition Master, why can't we find the desired drive letter in the drive letter dropdown list?
A: Please check whether another partition or CD-ROM device has occupied the desired drive letter. If yes, you can change the current drive letter of that partition or CD-ROM device to other one, and then assign it to the desired partition.
To change the drive letter, you can refer to:
Right click "My Computer" and choose "Manage" ->"Storage" -> "Disk Management", right click the partition or CD-ROM which has occupied the desired drive letter, and then choose "Change Driver Letter and Paths".