sreak's post proposing the katana devkit he owned gave me the idea to see with Laurent Comby, Swiss collector of many Dreamcast prototypes, if he did not have devkits pending.
Site I knew that on his (https://www.dreamcastsegainfo.org/) he offered one from Apaloosa (studio which made Ecco the Dolphin) with among others prototype versions of Ecco 1 and 2 which could not be finalized.
I had the good surprise that he offered me 2 other devkits. A devkit from Eurocom and another unknown (only the katana devkit ?) offered by retro gamer 4ever.
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For the Apaloosa devkit :
Hard disk image without editing : https://mega.nz/file/8o5DgCBI#xjvvtZ1KN ... 0tv0bHII-c
Image of the hard disk after using the Rstudio program : https://mega.nz/file/d9gRia6D#-YhdamWV1 ... Hy_GsBvJng
Different versions of Ecco 1 :
https://mega.nz/file/UowlxSga#G2IDv8Ox8 ... h9VkOYOG78 (does not work, the game starts but remains blocked, to be fixed)
https://mega.nz/file/4xoTRCAB#EBZGOEs6V ... UrzFpYIRoM (does not work, the game starts but remains blocked, to be fixed)
https://mega.nz/file/w95BFKRC#CLJWYbJRW ... tewENpGX_A (does not work, the game starts but remains blocked, to be fixed)
https://mega.nz/file/88hh3KYa#_rKgHl8MG ... 8GXcG0LQcs (does not work, the game starts but remains blocked, to be fixed)
2 versions of Ecco 2 :
one of the builds has the same version and same date as the ecco 2 found in 2016. However, when browsing the files, we can see some differences. The game is launched only on an emulator (to be fixed), map 1 does not work, the load remains blocked.
The other build has the same version and same date as the ecco 2 found in 2016 as well as the ecco 2 above. The differences are notable as new levels. Unfortunately, it crashes when loading one of the unpublished maps. To see to load the game on the title screen or on another map. Build to fix if possible.
build 1 : https://mega.nz/file/s8gxnQYT#6WTYvLZu0 ... -_CUdMoitA
build 2 : https://mega.nz/file/B5w1gSob#wsibxb0sj ... WhliNEm4-Q
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For the Eurocom devkit :
Hard disk image without retouching : https://mega.nz/file/JF4VTaSb#ZV3v2TJpL ... lY7LYF5nBY
Image of hard disk after using Rstudio program : https://mega.nz/file/MF4ziSTS#Jgaxe3qOX ... bCfkJlz5_E
Other hard drive image without retouching : https://mega.nz/file/BFgR0QAQ#qOAnoGBV0 ... Y_HDwps594
It would contain an NBA Hoopz prototype and a Hydro Thunder prototype.
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For the unknown devkit (thanks to retro gamer 4ever) :
Hard drive image without retouch : https://mega.nz/file/tVxjVA5D#d1nGVjTk3 ... -V7aRmU78Q
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I hope someone can bring out some prototypes or even some new games

Edit : Shame on me, I forgot to thank Laurent Comby of course

Edit 2 : the 3rd devkit apparently only contains the SDK