In some situations it is not known at profile creation time what drive letter Windows will assign to an external drive, for example, when it is plugged in. To get around this the variables SERIAL or LABEL can be used. For example, if your external drive has a serial number of ABCD-EFGH then you could set your source or destination to:
As an alternative the label variable can be used when the drive label is known, e.g.
When the profile is run SyncBackSE/Pro will substitute the variable with the drive letter of the drive with that serial or label, e.g.
An easy way to get the serial number, or label, or a drive is to use the Alternatives button in either the New Profile wizard or in the Profile Setup window if the profile has already been created. As an alternative, to get the serial number of a drive you could go to the command prompt and type dir x: (use the appropriate drive letter). The serial number of the drive is displayed in the header of the output. Note that the SyncBackSE/Pro log file also includes the serial number of the source and destination.
Note that neither SERIAL nor LABEL include the A or B drives (typically floppy disk drives). To include those you must use the SERIALALL and LABELALL variables. See the help file for more details.
This article explains how to assign permanent drive letters to external devices.