First of all I want to say that I have confidence in diagnosis which was not carried out in our Service Centre only if it was made by those experts I know personally. Because a great many of SC has multiplied but few of them have knowledge and skills. They often send or bring drives to our lab accompanying them with stories of diagnosis in other places. More than in a half of cases the results of previous “diagnosis” never confirmed.
However, if we suppose that an operation of head assembly replacement is necessary for data recovery from Toshiba drive from your Transcend box, then in case with Toshiba disks as in overwhelming majority of other cases complete coincidence of model is enough. So you can advise your experts to look for HDD MK5059GSXP or you can find it by yourself for example ordering the device on an online auction site e-bay.
In conclusion, I can assure you that you shouldn’t puzzle over finding necessary donor disk for your out-of-order storage recovery in case you apply to our Service Centre. All donor drives and other equipment required to accomplish the task are always available.