是的，大陆的「大佬」是从香港文化中借来的，而不是沿用古文的，也不是从台湾文化中借来的。I have no idea why 在大陆这个词一般写作“大佬”. I'm curious as well (Perhaps borrowed from Hongkong?).
大老 means 年高德劭者, 於一領域舉足輕重者, 高官元老, 幫會黨派長老, etc. It is the standard form used in the Classical Chinese literature (e.g., 孟子離婁上：「二老者，天下之大老也」) and 白話 literature of Yuan-Ming-Qing dynasties, and it continues to be so in Taiwan Mandarin.
佬 came from Cantonese, meaning "adult males". Phrases in Taiwan Mandarin that contain "佬" often carry a hint of derogatory undertone (e.g., 闊佬, 鄉巴佬). 大佬 sounds somewhat like 頭目--I mean, there is some potential negativity like English "alpha male", which may have an "animal, dehumanized" sense. 國民黨大老 is a neutral term, free of judgment; 國民黨大佬, on the other hand, may possibly come across as having prejudgment (like 共黨頭目). To be safe, use “大老” not "大佬” in Taiwan.
I have no idea why 在大陆这个词一般写作“大佬”. I'm curious as well (Perhaps borrowed from Hongkong?).
They mean the same--no difference in meanings. But 大佬 is not the usual form and therefore is more likely to evoke special connotations, for instance, 大佬文化 and 幫派文化 where hierarchy trumps democracy. It has something to do with collocation (e.g., when 大佬 often comes with the name of an organized gang (e.g., 竹联帮), the high frequency would naturally establish a connotation).你的意思是说国民党大老和国民党大佬在台湾有实质性的区别，太不可思议了吧。