Better is do clean - u will have that 100% perfect install.
1.Remove MSPD card and format it in PC
2.On PSP do Factory reset
3.On MSPD card on PC copy OFW and CFW files
4.Put MSPD card back to PSP
5.Install (Connect power suplay PSP and charged battery to 100%)
1.OFW
2.CFW
6.End
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Also on 5.50GEN-D remeber to change the version.txt inn flash from 9.90 to 5.50 and save. Before install newest FW (Becouse if not will show u have 9.90 FW and will not allow update becouse will think is newest version then u wonna update)
video how to do :
https://www.youtube.com/watch?v=LahsJj1Berg